The present invention relates to methods and apparatus for reclaiming contaminated liquid.
In particular, the present invention relates to a method and apparatus for reclaiming seal oil contaminated with H.sub.2 S.
Certain types of machines, such as centrifugal compressors, are required to process a gas which is sour, which is to say a gas which contains H.sub.2 S. Such machines include rotary shafts which must be sealed and lubricated. When the oil which leaks from such seals contains H.sub.2 S resulting from contact with the process gas and is reused without being purified, it is possible for the H.sub.2 S in the oil to cause a serious corrosion attack on babbitt-lined seal rings, thus drastically limiting the compressor run lengths.
The majority of centrifugal compressors in process gas service employ two liquid-film seals, each of which consists of two floating, non-rotating sleeves surrounding the compressor shaft with a close clearance. The sealing liquid is usually a compressor lube oil injected between the sleeves and flowing along the shaft so as to seal the extremities. The sealing liquid which passes through the inner of the two sealing rings, in other words the rings which face the interior of the casing of the compressor, comes into contact with the process gas and in so doing absorbs some of the gas. If this gas which contacts the sealing liquid contains H.sub.2 S, then this sealing liquid becomes corrosive and may attack the babbitt lining of the compressor seals and other equipment in the oil supply system. Thus, in the case of combined lube and seal oil systems, corrosive attack may extend to compressor bearings as well. In any event, corrosive attack on seals or bearings can have consequences ranging from excessive seal oil leakage to catastrophic failure of major equipment.
Corrosive damage to babbitt-lined components apparently is a function both of the metallurgical composition of the babbitt and the oil film operating temperature. These factors explain why some compressor seal designs are highly sensitive to H.sub.2 S-containing lube and/or seal oils while others are not. However, there is in any event an incentive to remove H.sub.2 S from seal oil even in apparently non-sensitive compressor installations because such removal will tend to reduce the formation of corrosion products due to interaction of H.sub.2 S and steel piping, thus reducing the formation of sludge in the oil reservoir.
Up to the present time, efforts to purity sour seal oil in an inexpensive manner have been unsuccessful. Nevertheless, reclamation of contaminated seal oil is highly desirable when the available alternatives are considered. Thus, it is possible to inject an inert buffer gas in a once-through manner into the space between the compressor casing interior and the inner sealing rings, but such a procedure involves undesirably high operating and maintenance costs.
If the contaminated seal oil is simply discarded, then it is necessary to replenish the discarded oil at an average rate of 20 gallons per stream day for compressors with seals which are in good condition, although severely worn seals have been known to leak at rates as high as several hundred gallons per day resulting, of course, in highly undesirable cost and disposal problems. Thus, there is not only a high cost for clean make-up oil, but in addition there is a serious environmental disposal problem.